Ore sorting in dependence on the radio active properties of ore particles is known. In the majority of known systems the ore particles to be sorted are fed, for reasons of sorting economy, at a fairly high speed past a crystal scintillation detector which is positioned to one side only of the path of the particles and which measures the radioactive emission from the particles. Difficulties with known systems are that, firstly, radio active emission from the particles is more often than not substantially directional due to emission shielding by the particle material and it is possible that a high grade particle may not be detected by the crystal or only detected at an erroneously low emission level and, secondly, the emissions from the particles are sporadic and may therefore not be picked up by the crystal detector as a particle is moved at high speed past it.
Both of these difficulties seriously impair the sorting efficiency of machines employing radio-active detection.